The present invention relates to an automatic frequency control circuit and, more particularly, to an automatic frequency control circuit in a receiver of time-division multiplex communication system.
An automatic frequency control circuit (AFC) in the prior art is constructed of a frequency converter for the frequency conversion of an input signal, a narrow-band filter connected to an output of said frequency converter, a phase comparator for comparing phases of input and output signals of said narrow-band filter and deriving at an output a voltage proportional to a phase difference between said input and output signals, and a voltage-controlled oscillator (VCO) supplied with the output of said phase comparator via a low-pass filter for delivering an output coupled to another input of said frequency converter. In such an AFC circuit, an input signal having a frequency offset is applied to the frequency converter to be mixed with the output of the voltage-controlled oscillator. The frequency of the output of the frequency converter is either a sum of or a difference between the frequency of the input signal and that of the output of the voltage-controlled oscillator. It is assumed here that the difference frequency is utilized in the system. The thus derived signal is applied to the narrow-band filter. A signal proportional to the phase difference between the input and the output signals of the narrow-band filter is derived from the phase comparator. This output signal is applied to the voltage-controlled oscillator via the low-pass filter, and thereby the voltage-controlled oscillator is controlled so that it may oscillate at a frequency having a fixed frequency difference with respect to the input signal.
If the input signal is incoming continuously as in the prior art, then the conventional AFC circuit fully achieves its function. However, in the case of the time-division multiplex satellite communication system in which a plurality of stations are linked on a time-division basis employing substantially equal frequencies, the modulated wave signals of the respective stations are received as intermittent signals, that is, as burst mode signals. In this case, if the AFC circuit is intended to respond to each burst from each station by employing the conventional circuit, then the cut-off frequency of the low-pass filter must be set to be high to shorten the response time of the AFC circuit. However, with such a cut-off frequency setting, thermal noise and other disturbances cannot be suppressed, so that the voltage-controlled oscillator is adversely affected. On the other hand, if the low-pass filter is given a low cut-off frequency, the AFC circuit cannot be fully responsive to a short burst signal even through it may be to a longer one.